Sometimes distinguished from fog, either as being less opaque or as consisting of drops large enough to have a perceptible downward motion. [OED]Also in Old English in sense of "dimness of the eyes, either by illness or tears," and in figurative sense of "things that obscure mental vision."
When misunderstanding serves others as an advantage, one is helpless to make oneself understood. [Lionel Trilling]
Meaning "dissention, disagreement" is first recorded 1640s.